Hello all, I'm trying to make a 8Mhz shortwave transmitter but I can't seem to get a valve to amplify 8Mhz,,, I can get them to amplify audio easy though so i'm guessing the frequency is to high for the valves? I'm using ecc81 valves I've also tried a big pl504 but it was not happy too

An ECC81 will happily amplify 8MHz or 80Mhz. I don't encourage unlicensed valve transmitters. A few low power transistors will be better, as a few mW won't do any harm. A valve transmitter will naturally produce nearly 1W without even trying.

Hi DF96, Wow 80Mhz is a lot. So I must be doing something wrong with the valve? I have thought about using transistors but I don't have any... oh 1 watt is a lot but I guess I could turn it's gain down to a few mw's .

Should I up load a diagram to see where I've gone wrong with the valve?

For the oscillator you can use some small cheap tube like 6BA6, with 1 MOhm or higher value of grid leak resistor, for grid auto-bias, between control grid and cathode. Wind some coil with about 100 turns on a pill cap, tap from 1/3. The tap goes to cathode, the lower end to the ground, from the upper end to the control grid through the cap like 51-100 pF. Add variable capacitor in parallel with the coil. Put some short antenna to the same upper end of the coil. Connect together screen grid an anode, and connect them to the anode of the SE amp. Turn it on, put some music, and wait for FCC and police to come and arrest you.